1. Field of the Invention
The present invention relates to an NC sentence creation system for creating an NC sentence for a two stage drilling operation, and more specifically, to an NC sentence creation system by which a machining time is shortened.
2. Background of the Related Art
FIG. 5 shows a tool feed operation when a two stage drilling operation is carried out by a numerically controlled machine tool using a conventional NC sentence First, a hole 32 is drilled to a workpiece 30 by a first stage drilling operation, then another not shown drill having a smaller diameter is moved by a quick traverse along a tool path 33a to the same point as that of the quick traverse stop point of the first stage drilling operation, i.e., to the point P30 which is spaced from the end 30a of a workpiece by an amount of clearance l31 and moved along a tool path 33b by a cutting feed to drill a hole 34.
Nevertheless, the drill does not make contact with the workpiece 30 from the point P30 to the bottom 32a of the hole 32 during the second stage drilling operation, and thus commanding the drill to be moved by the cutting feed speed in this area would be a waste of machining time.